The invention relates to a circuit for signal processing, for which a signal voltage, representing the signal, can assume a positive or negative value range or a value range of equal value relative to a reference potential, the circuit including an operational amplifier on the input side, the output of which is fed back to the inverting input and the noninverting input of which represents the input for the signal voltage, a differential amplifier, located between the inverting and the noninverting inputs, being formed by field-effect transistors, and a circuit for supplying the operational amplifier with an operating voltage.
As a rule, analog signals are voltages, which are bipolar with respect to a reference potential. Within a circuit system, ground potential (0 volt) is usually selected as the reference potential, a supply voltage of the system usually being constructed as a symmetrical supply with a positive and a negative operating voltage. The voltages of the analog signals accordingly assume a value, which, relative to the reference potential, lie in a positive or an equally large negative range of values.
In order to be able to process such voltages in other systems, particularly in measuring systems, two operating voltages, namely a positive and a negative operating voltage, are also usually provided in these circuit systems. Making these two operating voltages available represents an additional expense, since the two voltages must both be stabilized and screened. Moreover, if the second operating voltage is produced by means of a bootstrap generator, additional disturbances may be caused.
One possibility for avoiding a second operating voltage consists of increasing the reference voltage of the input voltage so that input voltages, below the ground potential, no longer occur. In principle, it would be possible to make available an internally produced reference potential at the input, onto which the analog signal is coupled capacitively. However, such a solution cannot be used for a d.c. voltage or for a low frequency application and thus does not represent a solution to the problem in most cases.
A different solution would consist of connecting a floating d.c. voltage in the input branch of the circuit arrangement. If, for example, a battery were to be used here, the overall size and the absence of long-term stability of this voltage source would be appreciably disadvantageous factors.